Transformer for connecting a wave guide to a coaxial line



Nov. 10, 1953 ALFORD 2,659,054

TRANSFORMER FOR CONNECTING A WAVE GUIDE TO A COAXIAL LINE Filed April 9, 1946 INVENTOR. ANDREW ALFORD av v ATTORNEY Patented Nov. 10, 1953 GUIDE TO A COAXIAL L Andrew Alford, Cambridge, Mass.,"assignor to the United States of Am the Secretary of War erica as represented 'by Application April9 1946, Serial No. 660,601 (01. 333-34) Claims. 1

This invention relates to electrical apparatus and more particularly relates to a coaxial line to waveguide transformer.

In transmitting R. F. energy through a system including waveguide means and coaxial transmission line means, it is necessary to use some kind of transforming device in order to have a good impedance match at the point of juncture. It is desirable that the frequency pass band of the system should not be limited in this transforming device. It is also desirable that the transforming device have a small coefilcient of reflection.

- In the prior art two methods of feeding a Waveguide from a coaxial transmission line, or of doing the converse are by the use of a radiating dipole which is joined to the line and placed inside the guide, and by the use of an inductive loop. Both of these methods have certain shortcomings as a result of which it is highly desirable that alternate or improved means be available. Dipoles used for this purpose areusually efiicient over frequency bands which are narrower than other parts of the system; they frequently are cramped for space inside of the guide; and they are frequently a mechanically fragile part of the system. Inductive loops are rather critical as to size and also have relatively narrow frequency pass bands. Moreover, small loop are more easily affected by becoming'corroded or dirty than a device made accordingto this invention.

It is an object of this invention to devise a transforming device Whichhas the desired electrical qualities of having a greatly increased -frequenoy pass band; a low coefficient of reflection; a balanced to unbalanced, or an unbalanced to balanced, transforming action; and a satisfactory impedance transformation ratio.

It is another object of this invention that this device be mechanically strong.

It is another object of this invention that this device be no more affected by ordinary wean-corrosion, and dirt, than ordinary coaxial transmission lines and waveguides generally.

Other objects, features, and advantages of this invention will suggest themselves to those skilled in the art and will become apparent from the following description of the invention taken in connection with the accompanying drawing in which:

Fig. 1 is an isometric view of one embodiment of this transforming device.

Fig. 2 is a schematic representation of a sectional view of the embodiment shown in Fig. 1,

2 said sectional view taken as indicated by arrows 2 in Fig. 1. g

Referring now more particularly to Fig. 1, waveguide I is terminated by end plate 2 to which is attached coaxial connector 3 which is attached in'a manner which will be explained in more detail below. The coaxial line which is to feed waveguide l, or to be fed by it, is not shown. It, however,'would be a conventional typeof coaxial transmission line which might have a characteristic impedance of 50 ohms. This coaxial line of course, would be attached to connector 3.

Side 4 is the broad top side of waveguide l for the position in which waveguide I is shown in the drawing. The side opposite to side 4 is not shown or numbered. This opposite side and side 4 extend all the way to the guide termination at end plate 2 with a distance between them which is maintained constant for the whole length. This distance between these sides is governed simply by the fact that itis one of the normal dimensions of the guide chosen for a particular system. Narrow side 5 and the narrow side opposite to it, which is not shown in the drawing, are constructed so that at a selected distance from end plate 2 they begin to taper gradually inward toward each other, instead of continuing to be parallel right up to end plate 2. As a result waveguide i has cross sections of progressively smaller widths as these cross sections are taken nearer and nearer to end plate 2.

Triangular conductive rib 6, shown inside waveguide I in dotted outline in Fig. 1, may be made of thin sheet metal. It has two edges which meet at right angles and a third edge which is related to the other two as the hypotenuse is related to the other two sides of a right triangle. One of the two edges which meet to form a right angle may be a few waveguide wavelengths long. This edge is mechanically and electrically attached to the inside wall of side 4 and extends longitudinally along a straight line down the center of this inside wall. Rib 6 is thereby attachedto said wall and projects inward at a substantially right angle to it. Rib 6 is so located along this center line that a short edge, which is at right angles to the edge which is fastened to side I, abuts against the inner surface of end plate 2 and is mechanically and electrically connected to it. Two of the three edges of rib 6 come together to form a very small angle located at the apex of the triangle formed by the edges of rib 6 which is opposite to the edge which abuts against end plate 2. The two edges which form this small angle both terminate 3 against the inner wall of side 4 at a point midway between its two edges. This is approximately the point along side 4 where side 4 and the side opposite to it begin to narrow because of the gradual inclination of narrow side 5 and the side opposite to it as described above. In other words,- the taper in the widthxor the guide starts at substantially the point along the guide where the apex at the end of rib 6 is fastened to the inside wall of side 4.

The edge of rib 6 which abuts against end plate 2 and is attached thereto ha'sa length "of about one-third of the distance between theimierwau of side 4 and the side opposite it. Within *a narrow range this length of this edge, and. accordingly the width of triangular rib 6, may be varied by trial and error, or otherwise, to-achieve certain electrical results which will beaessribea hereinafter.

Metallic tube 1 is indicated in Fig. l by parallel dotted lines. lt-is asection of tubing mechanically and electrically affixed to the edge of rib 6 which lies along the hypotenuse of the right triangle formed by its three edges. It is obvious that if tube I were to have exactly the same length as the edge of rib 6 to which it i attached, tube 1 would terminate abruptly where said edge meets the inner wall of side 4. To avoid this, tube -'1 is not cut off squarely at this point. instead it is cut obliquely along its intersection with aplane which is intercepted by the axi of tube I at an angleequal to the smallest angle in the triangle formed by the edges of rib 6. Therefore, in effect, a side of tube 7 which is opposite to the side whereit is attached to rib 6, extends further into the guide than its side which is so attached. And, as a further result, this end'of tube -1 will have a long narrow eliptical opening. This long narrow opening should bemade toabut againstthe inner wall of side i and should be electrically and mechanically attached thereto.

Because the long edges ofrib 6 are nearly parallel, tube 1 may be cut off substantially squarely at its opposite end. That end, however, should be so cut that it will abut tightly against the inside of end plate 2 and it should be electrically and mechanically connected thereto. These various connections -may be made by soldering with soft solder, hard solder, or silver solder; by-brazing; or by any oneof a wide variety of conventional means.

A triangular rib 8 whichcorresponds to rib 6 and a tubular element 9 which corresponds to tube 1 are connected along the center line of the opposite inner wall ofthe guide I and to end platez in a mannercorrespondling exactly to that described in detail above with respect to rib 6, tubular element 1, side 'I, and end .plate -2.

It is obvious that if the manner of construe tionherein set forth is followed, tubular elements I and 9 will be separated by increasingly smaller distances as these elements approach nearer to end plate 2. -At end plate-2 they will be separated by a distance of about one-thirdof the thickness of the guide as measured inside it.

I-n-amodel which was made and tested by the inventor and was used in connection with a transmission system including 'a waveguide 10.3 centimeters wide and 5 centimeters thick the following dimensions were employed: the taper at the end of the guide narrowed it down to a width of 6 centimeters at the endplate; the beginning of the taper was at a point 3 wavelengths away structure which will be described in more detail below, was located at a point one-quarter of a wavelength inside the waveguide as measured from'tne end plate. This device operated in a very satisfactory manner over a wide frequency band. It'was determined that none of the dimensions "was critical. For these dimensions the coaxial cable used was standard stock material known as'R'G 9U solid dielectric cable.

Endplate 2 has a hole cut through it at the point where tubular element 9 abuts against it so that the opening-in thiselement is continued on through'end plate 2. Connector '3 is fastened to the outside of end plate '2 so that it-is directly over this hole. The inner conductor of connector 3 is mechanically and electrically con nected to an inner conductor it which extends inward within tube 9 from connector 3 fora distance of about one-quarter of a wavelength. It is insulated from tube 9 by any appropriate means such as a dielectric coating around inner conductor it. At a distance of approximately one quarter wavelength from end plate 2 there is a hole in tube 9 on its side nearest to tube '1. This hole should be large enough to permit-innerconductor 10 to be led-out of tube -'9 without coming into electrical contact with the edgesof said hole. Said inner'conductor is led for a short distance across the guide to a point-on. the side of tube 1 which is opposite to said hole in tube 9. Inner conductor It is mechanically and-electrically connected to tube 1 at that point.

Referring more particularly to Fig. 2, the arrangement of inner conductor it with respect to connector 3 and tubes 1 and 9 is shown'more clearly than by the dotted lines of Fig. -1.

The foregoing portion of this application has described in detail the physical construction of one embodiment of this invention. The portion which follows will explain the principles of this invention.

It will be seen that the "portionsoi tubes 53 and l occupying the spaces respectively lying between the hole on tube-9 and end plate Land the point on tube I where inner conductor H1 is connected to it and end plate 2, together with inner conductor I6 and connector 3, constitute a coaxial line to waveguide coupler or transformer.

Devices for connecting a coaxial line, or other line which is unbalanced toground, to a balanced line is called a balun. Asin an ordinary balun,

unbalanced R. F. energy can be fed into connector 3 and transformed into a balanced condition across the two tubular elements 1 and 9. If the length of the balu'n is properly selected "with respect to a given operating wavelength, this selected length being in any case generally of the order of one-quarter of said'wavelength, a coaxial feed line attached to connector twill encounter aproper impedance match. For practical purposes, it may be assumed that the impedance across the balun looking back in the 'di rection of the coaxial line, and measured across the points where inner conductor l0 bridges from the end plate; theribs and their associated -across'tubes 1 and 9, is fibbh ms. "That 'to say,

it is substantially the same impedance as a characteristic impedance of the coaxial line.

However, the waveguide ordinarily will have a characteristic impedance (in the transmission line sense) which is somewhat higher than that of such a coaxial line. Therefore, some impedance transforming means is needed. This means is provided in ribs 6 and 8 and their associated elements, tubes 1 and 9. These elements capacitively load waveguide l. The reason for the gradual taper in these loading elements is to prevent a sudden change in the characteristic impedance of the guide. If the taper of these loading elements is made as gradual as the taper described herein, the transformation will be in practical effect, nearly as gradual as if the taper were extended over an infinite length of guide.

The reason for narrowing the guide simultaneously with increasing the guide loading, that is, as the ribs become progressively wider and wider at points nearer and nearer to the end plate 2, is rather obvious. According to some concepts, a waveguide can be considered as a transmission line of the parallel-pair variety in which the parallel pair is shunted by many anti-resonant tuned stubs in such close juxtaposition on both sides of it as to be actually contiguous along the entire length of said pair. According to this theory the physical portion of the guide structure which would correspond to the said imaginary parallel pair would be, for the TEm mode of operation, two central parallel longitudinal strips lying along the centers of opposite inner walls of the wide sides of the guide. An' anti-resonant stub has to be physically reduced in length if it is capacitively loaded and thereby electrically lengthened. Therefore, the guide is narrowed progressively as it is loaded more and more. If a correct amount of gradual capacitive loading is used, that is, if ribs 6 and 8 and tubes 1 and 9 are made to approach each other closel enough, the portion of this device which has been described as corresponding to a balun will see an impedance, looking down the guide in the direction away from end plate 2, which is lower than the characteristic impedance of the waveguide system generally and which will be a proper match for the impedance of this balun portion.

It is obvious that as a result of this design this transformer means offers a proper coaxial line to waveguide impedance match. Moreover, this device is very broad banded due to the fact that the balun, which is the limiting integral portion of it, is very broad banded. This device also offers an excellent unbalanced to balanced transformation and its coeflicient of reflection is very small.

Obviously energy can be transmitted through this apparatus in either direction and the same beneficial results will be obtained.

While there has been here described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention.

What is claimed is:

1. A transformer for matching a coaxial line having a. first characteristic impedance to a rectangular waveguide having a second characteristic impedance, said transformer comprising a waveguide portion having a rectangular crosssection, said waveguide portion having a length of at least one wavelength and a width and depth at one end thereof equal respectively to the width and depth of said rectangular waveguide, the width of said waveguide portion being symmetrically tapered to provide a width at the other end thereof which is less than the width of said rectangular waveguide, first and second identical approximately right triangular members each having a first leg which is approximately one wavelength at the operating frequency and a second leg which is less than one-half the depth of said rectangular Waveguide, said first legs attached respectively to the upper and lower wide walls respectively of said waveguide portion, said first legs being in coincidence with the respective center lines of said wide walls and said second legs being in coincidence with said other end of said waveguide portion, said second legs lying on a line parallel to the narrow walls of said waveguide portion, said waveguide portion being connected to said rectangular waveguide at said one end thereof, and said coaxial line being connected to said first and second members.

2. A transformer as defined in claim 1, wherein each of said first and second members comprise a right triangular metallic plate and a metallic tube attached along the hypotenuse of said metallic plate member.

3. A transformer as defined in claim 2, wherein said coaxial line has its outer conductor connected to one of said tubes at the junction of the second leg and hypotenuse of one of said members, and its inner conductor disposed adjacent to but insulated from the other of said tubes for a distance of approximately one-quarter wavelength at the operating frequency from the junction of the second leg and hypotenuse of the other of said members, said inner conductor then being bent into a loop terminating on said one tube at a distance of approximately one-quarter wavelength at the operating frequency from the junction of the second leg and hypotenuse of said one member.

4. A transformer as defined in claim 1, wherein said second leg i approximately equal to onethird of the depth of said rectangular waveguide.

5. A transformer as defined in claim 1, further including a metallic plate disposed across said other end of said waveguide portion.

ANDREW ALFORD.

References Cited in the file of this patent UNITED STATES PATENTS France Mar. 7, 1944 

